A radiation therapy device controller identifies a radiation irradiation position in a living body based on a position of a diseased portion (tumor) displayed on a CT image (a computed tomography image) and transmits the position to the radiation therapy device. Accordingly, the radiation therapy device irradiates a diseased portion in the living body with radiation based on a radiation irradiation position received from a radiation therapy device controller to perform treatment. Here, a position of the diseased portion such as a tumor in the living body varies due to influence of respiration or the like. Accordingly, it is necessary to accurately track the position of the diseased portion varying in the living body in order to improve the accuracy of irradiation of radiation to an irradiation position of the diseased portion. In Patent Document 1, technology for designating a diseased portion area in a radiation projection image (transmission image) and irradiating the diseased portion with radiation based on the diseased portion area is described (See Patent Document 1).